Recently, accompanying the increased speed of data transmission, the application of optical signals, which have minimal deterioration even with high speed transmission over long distances, has replaced electrical signals that, conventionally, have been widely used as a data communications medium. When high optical signal quality is demanded such as with long distance optical transmission, an external modulator yielding high signal quality is often used as an optoelectronic converting device. However, schemes of using an external modulator have a problem of increased cost and consequently, for short distances, direct modulation is employed, where an optoelectronic device (light emitting device) such as a laser diode (LD) is directly turned ON and OFF to convert an electrical signal into an optical signal.
When an LD is used for direct modulation with an electrical signal, even if an electrical signal having symmetrical rising and falling edges is input, consequent to the properties of the LD, such as relaxation oscillation, the LD output is a waveform having asymmetrical rising and falling edges, arising in a problem of degraded transmission characteristics. Therefore, asymmetrical pre-emphasis is used, where, at the electrical signal stage, by preliminarily emphasizing the falling edge (falling edge peaking), which significantly degrades compared to the rising edge, the degradation is compensated (for example, refer to Japanese Laid-Open Patent Publication No. 2006-40974).
In the case of direct modulation using an LD and an electrical signal, from the perspective of optical signal quality improvement and high speed operation, asymmetrical pre-emphasis of not only emphasizing the falling edge of the signal, but also preliminarily emphasizing the rising edge of the signal (rising edge and falling edge peaking) is also employed. In this case as well, like the technique in Japanese Laid-Open Patent Publication No. 2006-40974, a technique is used where the emphasis of the falling edge becomes particularly great consequent to the rising edges and falling edges being asymmetrical.
Nonetheless, with the conventional techniques above, the circuit configuration implementing asymmetrical pre-emphasis is complicated, whereby the scale of the drive circuit becomes large and parasitic capacitance increases making high speed operation difficult. The increased scale of the drive circuit results in a further problem of increased power consumption.